Diffractive optical element and optical system using the same

ABSTRACT

A diffractive optical element includes stacked first and second diffraction gratings made of different materials. The materials of the first and second diffraction gratings are glass. The first and second diffraction gratings have grating surfaces contacted to each other. The materials satisfy a predetermined condition when Tg 2  and At 2  are a transformation point temperature and a yield point temperature of the material of the first diffraction grating, and Tg 3  and At 3  are a transformation point temperature and a yield point temperature of the material of the second diffraction grating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffractive optical element suitablefor an optical system applied to a video camera, a digital camera, asilver-halide film camera, or the like.

2. Description of the Related Art

As a method for reducing chromatic aberration of a lens system by acombination of glass members, a method is known in which a diffractiveoptical element having diffraction effect is provided at a surface of alens, or at a part of a lens system, to reduce chromatic aberration ofthe lens system (see SPIE Vol. 1354 International Lens Design Conference(1990), U.S. Pat. No. 5,044,706, U.S. Pat. No. 5,790,321, and U.S. Pat.No. 5,044,706).

The method adopting the diffractive optical element uses a physicalphenomenon in which chromatic aberration caused by a light beam with areference wavelength through a refractive surface is a reversal ofchromatic aberration through a diffractive surface.

The diffractive optical element may serve as an aspherical lens bychanging the period of the periodic structure. Hence, the diffractiveoptical element can also reduce other aberration in addition to thechromatic aberration.

In an optical system having the diffractive optical element, if majorlight beams with usable light wavelengths concentrate at diffractionlight with a predetermined order (hereinafter, referred to as “givenorder” or “design order”), the intensity of diffraction light with otherorders may be low. If the intensity is zero, it theoretically means thatthe diffraction light does not exist.

In fact, unwanted diffraction light with orders except for the designorder exists. The unwanted diffraction light passes the optical systemthrough a passage different from that of the light beam with the designorder, causing flare.

To reduce the aberration using the diffractive optical element, thediffraction efficiency of the diffraction light with the design orderhas to be sufficiently high for the entire usable light wavelengths.

Also, it is important to carefully consider the spectral distribution ofthe diffraction efficiency of the diffraction light with the designorder, as well as the behavior of the unwanted diffraction light with anorder except for the design order.

A diffractive optical element that improves the diffraction efficiencyand reduces the unwanted diffraction light is suggested (see U.S. Pat.No. 5,847,877, U.S. Patent Application Publication No. 2003-0161044,U.S. Patent Application Publication No. 2006/0171031, U.S. Pat. No.6,157,488, and U.S. Pat. No. 6,560,019).

The diffractive optical element, disclosed in U.S. Pat. No. 5,847,877,U.S. Patent Application Publication No. 2003-0161044, or U.S. PatentApplication Publication No. 2006/0171031, has two contacting diffractiongratings, and the material and height of the diffraction gratings areappropriately determined (hereinafter, such a diffractive opticalelement (DOE) is referred to as “contacting two-layer DOE”).

Accordingly, high diffraction efficiency of diffraction light with adesired order can be provided for wide wavelengths. It is noted that thediffraction efficiency is expressed by the ratio of the light quantityof diffraction light with an order to the light quantity of the entiretransmitted light.

A diffractive optical element, disclosed in U.S. Pat. No. 6,157,488 orU.S. Pat. No. 6,560,019, has a plurality of stacked diffractiongratings, and the material and height of the diffraction gratings areappropriately determined (hereinafter, such a diffractive opticalelement (DOE) is referred to as “stacked DOE”).

Accordingly, high diffraction efficiency of diffraction light with anyorder can be provided for wide wavelengths.

U.S. Pat. No. 5,847,877 discloses a contacting two-layer DOE in whichdiffraction gratings made of two kinds of glass members are stacked.

U.S. Patent Application Publication No. 2003-0161044, U.S. PatentApplication Publication No. 2006/0171031, or U.S. Pat. No. 6,157,488discloses contacting two-layer DOE or a stacked DOE in which twodiffraction gratings made of two kinds of UV curable materials arestacked.

U.S. Pat. No. 6,560,019 discloses a stacked DOE using a glass member andUV curable resin.

Also, a contacting two-layer DOE or a stacked DOE is known in which amaterial of a grating surface (diffraction grating) is the same materialas a substrate on which the diffraction gratings are provided.

For a contacting two-layer DOE or a stacked DOE, it is important toappropriately determine materials of a plurality of diffractiongratings, and to appropriately determine a method of manufacturing thediffraction gratings, so as to have high diffraction efficiency and highenvironment resistance for wide wavelengths.

If the manufacturing method is not appropriately determined especiallydepending on the kind of the material; it is difficult to obtain astacked DOE having high diffraction efficiency and high environmentresistance for wide wavelengths.

U.S. Pat. No. 5,847,877 discloses an embodiment of the diffractiveoptical element, in which diffraction gratings made of two kinds oftypical glass members are contacted, however, does not disclose aspecific manufacturing method of the diffraction gratings.

To manufacture the diffraction gratings with typical glass members usinga mold, since the typical glass members have a yield point temperatureof 600° C. or higher, the molding temperature has to be 600° C. orhigher. The durability of the mold may decrease as the moldingtemperature increases, and hence, the productivity may decrease.

A grating pattern may be formed by directly cutting glass, or bylithography and etching. The manufacturing method may be complicated,and hence, the productivity may decrease. In addition, U.S. Pat. No.5,847,877 does not disclose a manufacturing method for contacting thetwo kinds of diffraction gratings together.

U.S. Patent Application Publication No. 2003/0161044, U.S. PatentApplication Publication No. 2006/0171031, U.S. Pat. No. 6,157,488, orU.S. Pat. No. 6,560,019, discloses a manufacturing method using resin asthe material of the diffraction gratings. If the resin is thermoplasticresin, a transformation point temperature thereof may be 200° C. orlower. The molding temperature may be low, and the productivity may behigh. If the resin is photocurable resin, the resin may be molded withexposure of light. Hence, the productivity may be high.

As described above, the productivity can be improved by using resin forthe material of the diffraction grating. However, the characteristic ofresin may be changed because of heat, humidity, and ultraviolet light.The environment resistance may decrease, and the application may berestricted. Also, when a mold is used for molding, a diffraction gratingmade of resin may be deformed because of shrinkage when being cured.

SUMMARY OF THE INVENTION

The present invention provides a diffractive optical element having highproductivity, high environment resistance, and high diffractionefficiency for wide wavelengths.

A diffractive optical element according to an aspect of the presentinvention includes stacked first and second diffraction gratings made ofdifferent materials. The materials of the first and second diffractiongratings are glass. The first and second diffraction gratings havegrating surfaces contacted to each other. The materials satisfy thefollowing conditions:Tg2≦600° C., andTg3≦600° C.,and the materials satisfy one of the following conditions:Tg2≠At3, andTg3≠At2,where Tg2 and At2 are a transformation point temperature and a yieldpoint temperature of the material of the first diffraction grating, andTg3 and At3 are a transformation point temperature and a yield pointtemperature of the material of the second diffraction grating.

A method of manufacturing a diffractive optical element includingstacked first and second diffraction gratings made of differentmaterials, according to another aspect of the present invention,includes molding the second diffraction grating using a metal mold at amolding temperature of a yield point temperature or higher of thematerial of the second diffraction grating; cooling the seconddiffraction grating to a transformation point temperature or lower ofthe material of the second diffraction grating while being contacted tothe metal mold, and removing the metal mold; molding the firstdiffraction grating using the molded second diffraction grating as aglass mold at a molding temperature in a range from the transformationpoint temperature or lower of the material of the second diffractiongrating to a yield point temperature or higher of the material of thefirst diffraction grating; and cooling the first and second diffractiongratings while being contacted to each other.

A method of manufacturing a diffractive optical element includingstacked first and second diffraction gratings made of differentmaterials, according to still another aspect of the present invention,includes molding the second diffraction grating using a first metal moldat a molding temperature of a yield point temperature or higher of thematerial of the second diffraction grating; cooling the seconddiffraction grating to a transformation point temperature or lower ofthe material of the second diffraction grating while being contacted tothe first metal mold, and removing the first metal mold; molding thefirst diffraction grating and a third diffraction grating at a surfaceopposite to the first diffraction grating using the molded seconddiffraction grating as a glass mold and a second metal mold for asurface opposite to the second diffraction grating at a moldingtemperature in a range from the transformation point temperature orlower of the material of the second diffraction grating to a yield pointtemperature or higher of the material of the first diffraction grating;cooling the first and second diffraction gratings and the second metalmold to a transformation point temperature or lower of the material ofthe first diffraction grating while being contacted to each other, andremoving the second metal mold; and cooling the first and seconddiffraction gratings while being contacted to each other.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a front view and a side view of a diffractive opticalelement according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the element structure of thediffractive optical element according to the first embodiment.

FIG. 3 is a graph showing diffraction efficiency of design orders ofpositive first-order diffraction light, zero-order diffraction light,and positive second-order diffraction light, of the diffractive opticalelement according to the first embodiment.

FIGS. 4A-F are explanatory diagrams showing the formation of adiffractive optical element using the manufacturing method of thediffractive optical element according to the first embodiment.

FIG. 5 is an explanatory diagram showing the outer diameter relationshipof the diffractive optical element according to the first embodiment.

FIG. 6 is an explanatory diagram showing the element structure of adiffractive optical element according to a second embodiment of thepresent invention.

FIG. 7 is a graph showing diffraction efficiency of design orders ofpositive first-order diffraction light, zero-order diffraction light,and positive second-order diffraction light, of the diffractive opticalelement according to the second embodiment.

FIG. 8 is an explanatory diagram showing the manufacturing method of thediffractive optical element according to the second embodiment.

FIG. 9 is an explanatory diagram showing the element structure of adiffractive optical element according to a third embodiment of thepresent invention.

FIG. 10 is a graph showing diffraction efficiency of design orders ofpositive first-order diffraction light, zero-order diffraction light,and positive second-order diffraction light, of the diffractive opticalelement according to the third embodiment.

FIG. 11 is a schematic illustration showing an image-forming opticalsystem according to a fourth embodiment of the present invention.

FIG. 12 is a schematic illustration showing an observing optical systemaccording to a fifth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Diffractive optical elements according to embodiments of the presentinvention are described below with reference to the drawings.

A diffractive optical element has a plurality of diffraction gratingsstacked on each other. The maximum difference in optical-path length oflight transmitted through the plurality of diffraction gratings issubstantially an integral multiple of the wavelength of the light.

The diffractive optical element has diffraction gratings of two or morelayers in which first and second diffraction gratings made of twodifferent materials are stacked.

The materials of the first and second diffraction gratings are moldingglass.

Grating surfaces of the first and second diffraction gratings arecontacted to each other.

First Embodiment

FIG. 1 includes a front view (illustrated on the left) and a side view(illustrated on the right) of a diffractive optical element according toa first embodiment. The location of an optical axis O of the diffractiveoptical element is illustrated for the side view.

FIG. 2 is a part of a cross section of the diffractive optical elementin FIG. 1 taken along line II-II′ in FIG. 1.

FIG. 2 is an illustration deformed along the grating height (depth) foreasy understanding of the grating pattern. In FIGS. 1 and 2, adiffractive optical element 1 has a first diffraction grating 2 and asecond diffraction grating 3, whose grating surfaces are contacted toeach other.

The first diffraction grating 2 is defined by a grating portion 2 b, andthe second diffraction grating 3 is defined by a grating portion 3 b.The grating portions 2 b and 3 b are arranged with a predeterminedpitch. The grating portions 2 b and 3 b of the first and seconddiffraction gratings 2 and 3 have a concentric grating pattern. Thegrating pitch of the grating portions 2 b and 3 b gradually decreasefrom the center (optical axis) toward the periphery of the gratingportions 2 b and 3 b, so as to provide a lens effect (convergence ordivergence). All layers of the diffraction gratings 2 and 3 serve as onediffractive optical element.

In this embodiment, while the diffractive optical element 1 isillustrated as a flat plate, the surface having the diffraction gratingsdoes not have to be flat and may be spherical, aspherical, or curved.

The diffraction gratings 2 and 3 satisfy d/P<1/6, where P (μm) is agrating pitch of the grating portions 2 b and 3 b, and d (μm) is agrating height (grating thickness) of the grating portions 2 b and 3 b.As long as the above condition is satisfied, the diffractive opticalelement or a mold for manufacturing the diffractive optical element canbe easily machined to have such a grating pattern.

The usable light wavelengths of the diffractive optical element 1 inthis embodiment are visible wavelengths. Hence, the material and gratingheight of the first and second diffraction gratings 2 and 3 are selectedso that the diffraction efficiency of positive first-order diffractionlight may be high for the entire visible wavelengths.

In particular, the material and grating height of the diffractiongratings 2 and 3 are determined so that the maximum difference inoptical-path length (the maximum value of the difference between anoptical-path length through the vertex of the diffraction portion and anoptical-path length through the root thereof) of light transmittedthrough the plurality of diffraction gratings (diffraction gratings 2and 3) is substantially an integral multiple of the wavelength of thelight within the usable light wavelengths.

The material and pattern of the diffraction gratings are properlydetermined, and hence, high diffraction efficiency can be provided forthe entire usable light wavelengths.

In this embodiment, the diffraction efficiency of a given order of thediffractive optical element for the entire usable light wavelengths is95% or higher.

Next, the diffraction efficiency of the diffractive optical element 1according to this embodiment is described.

When the two diffraction gratings 2 and 3 are contacted to each other toform a contacting two-layer diffractive optical element (DOE), thecondition for the maximum diffraction efficiency of certain-orderdiffraction light with a design wavelength λ0 is determined such thatthe difference between the optical-path length through the vertex of thegrating portion and the optical-path length through the root thereof(the difference between the optical-path length passing through thevertex of the grating portion and that passing through the root thereof)is obtained for all diffraction gratings; the values are added; and thenthe sum is substantially an integral multiple of the wavelength.Accordingly, in the diffractive optical element 1 of this embodimentshown in FIGS. 1 and 2, the conditional expression for the maximumdiffraction efficiency of the diffraction light with the designwavelength λ0 and a diffraction order m is as follows:±(n01−n02)d=mλ0  (1),where n01 is a refractive index with the wavelength λ0 of a material ofthe first diffraction grating 2, n02 is a refractive index with thewavelength λ0 of a material of the second diffraction grating 3, d is agrating height of the first and second diffraction gratings 2 and 3, andm is a diffraction order.

Herein, it is assumed that a light beam diffracted downward with respectto the zero-order diffraction light in FIG. 2 has a positive diffractionorder, whereas a light beam diffracted upward with respect to thezero-order diffraction light in FIG. 2 has a negative diffraction order.Concerning the positive or negative sign of the grating height in theexpression (1), it is assumed that the relationship between refractiveindices n01 and n02 of materials 2 a and 3 a for the diffractiongratings 2 and 3 is n01<n02.

When the grating height of the grating portions 2 b and 3 b increasesfrom a lower portion to an upper portion in FIG. 2, the sign becomesnegative. In contrast, when n01>n02, and the grating height of thegrating portions 2 b and 3 b decreases from the lower portion to theupper portion in FIG. 2, the sign becomes positive. With the structurein FIG. 2, when the relationship between the refractive indices n01 andn02 is n01<n02, the expression (1) can be rewritten as follows:−(n01−n02)d=mλ0  (2)

With the structure in FIG. 2, a diffraction efficiency η(λ) with awavelength λ, which is not the design wavelength λ0, can be expressed asfollows:

$\begin{matrix}\begin{matrix}{{\eta(\lambda)} = {\sin\; c\;{2\left\lbrack {\pi\left\{ {M - {\left\{ {{- \left( {{n\; 01(\lambda)} - {n\; 02(\lambda)}} \right)}d} \right\}\text{/}\lambda}} \right\}} \right\rbrack}}} \\{= {\sin\; c\;{2\left\lbrack {\pi\left\{ {M - {{\phi(\lambda)}/\lambda}} \right\}} \right\rbrack}}}\end{matrix} & (3)\end{matrix}$Herein, φ(λ) in the expression (3) can be expressed as follows:φ(λ)=−(n01(λ)−n02(λ))d  (4)

In the above expressions, M is the order of diffraction light to beevaluated, n01(λ) is a refractive index with the wavelength λ of thematerial 2 a of the first diffraction grating 2, n02(λ) is a refractiveindex with the wavelength λ of the material 3 a of the seconddiffraction grating 3, and d is the grating height of the gratingportions 2 b and 3 b.

Next, an implemental diffractive optical element 1 as an example isdescribed below in more details to explain the feature of the firstembodiment. In this embodiment, the materials 2 a and 3 a of the firstand second diffraction gratings 2 and 3 employ molding glass. Themolding glass is low-melting glass having a yield point temperature Atof 600° C. or lower.

The material 2 a of the first diffraction grating 2 employs precisionmolding optical glass, named K-PG395 (nd=1.658, νd=36.9, transformationpoint temperature Tg=363° C., yield point temperature At=392° C.),manufactured by Sumita Optical Glass, Inc.

The material 3 a of the second diffraction grating 3 employs precisionmolding optical glass, named K-VC80 (nd=1.694, νd=53.1, Tg=530° C.,At=566° C.), manufactured by Sumita Optical Glass, Inc.

The total grating thickness of the grating portions 2 b and 3 b of thefirst and second diffraction gratings 2 and 3 is 20 μm or smaller.

It is assumed that the grating height d is 16.55 μm.

FIG. 3 shows the characteristic of the diffraction efficiency of thediffractive optical element with the design order (positive firstorder), and the characteristics thereof with the zero order and apositive second order, which are ±1 order with respect to the designorder.

The diffraction efficiency of the diffraction light with the designorder is 95.0% or higher for the entire visible wavelengths. Hence,unwanted flare can be 1.0% or lower for the entire visible wavelengths.

Herein, the diffraction efficiency with unwanted order light is obtainedonly for the zero-order and the positive second-order diffraction lightbecause the diffraction order far from the design order may cause lessflare. If the flare with the zero order and the positive second order,which are close to the design order, is reduced, flare caused byhigher-order diffraction light can be also reduced. The diffractiveoptical element designed to mainly diffract light with a predetermineddesign order may have diffraction efficiency which decreases as theorder comes far from the design order. The order far from the designorder may cause blurring in an image plane, thereby the flare becomingless noticeable.

The characteristics that may vary due to environmental change in theexpression (3) for the diffraction efficiency η(λ) are the refractiveindex of the material and the grating height. In particular, thevariation in refractive index due to the environmental change is avariation in refractive index dn/dt due to temperature change. Thevariation in grating height of the grating portion depends on expansion(linear expansion coefficient) due to temperature change and expansion(swelling index) due to humidity change. The variation in refractiveindex dn/dt, linear expansion coefficient, and swelling index of glassare about 1/10 as compared with values of resin.

The diffraction gratings 2 and 3 of the diffractive optical element 1according to the first embodiment are made of only the glass moldingmaterials. Therefore, with this embodiment, the variation in refractiveindex dn/dt, linear expansion coefficient, and swelling index can bemarkedly reduced as compared with the values of resin. The variation indiffraction efficiency due to the environmental change can be markedlyreduced.

Also, since the diffraction grating is molded using the glass moldingmaterial, a diffraction grating with an accurate pattern can be moldedas long as the grating height d of the grating portion can be reduced.To reduce the grating height of the grating portion, the Abbe number νdof the material of at least one of the first and second diffractiongratings 2 and 3 may be 40 or smaller.

In addition, the Abbe number νd of the material of at least one of thefirst and second diffraction gratings 2 and 3 may be 50 or greater.

In particular, the Abbe number of the glass molding material for formingthe first diffraction grating 2 may be 40 or smaller, and the Abbenumber of the glass molding material for forming the second diffractiongrating 3 may be 50 or greater.

In the above-described first embodiment, while the diffractive opticalelement employs first-order diffraction light with a positive firstorder as the design order, the design order is not limited to thepositive first order. Advantages similar to those of the firstembodiment may be provided as long as the difference in the optical-pathlength of the diffractive optical element is determined to correspond toa predetermined design wavelength with a predetermined design order evenwith a positive second order, a positive third order, or the like.

Next, a manufacturing method of the implemental diffractive opticalelement 1 is described below in details to explain the feature of thediffractive optical element 1 of the first embodiment.

The materials of the first and second diffraction gratings 2 and 3 usedin this embodiment have the following characteristics.

It is assumed that Tg2 is the transformation point temperature and thatAt2 is the yield point temperature of the material of the firstdiffraction grating 2.

Also, it is assumed that Tg3 is the transformation point temperature,and that At3 is the yield point temperature of the material of thesecond diffraction grating 3.

The following conditions are satisfied:Tg2≦600° C., andTg3≦600° C.Also, one of the following conditions is satisfied:Tg2≠At3, andTg3≠At2.

In particular, the first and second diffraction gratings 2 and 3 in thisembodiment may employ the materials satisfying one of the followingconditions:Tg3>At2, andTg2>At3.

More particularly, the materials may satisfy one of the followingconditions:Tg3−At2>50° C., andTg2−At3>50° C.

Further particularly, the materials may satisfy one of the followingconditions:Tg3−At2>100° C., andTg2−At3>100° C.

In this embodiment, the materials satisfy the following conditions fromamong the above-mentioned conditions:Tg3>At2, andTg3−At2>100° C.

Now, the manufacturing method of the diffractive optical element usingthese materials is described.

The diffractive optical element 1 in this embodiment has a structure inwhich the first and second diffraction gratings made of differentmaterials are stacked.

First, the second diffraction grating is molded using a metal mold at amolding temperature of the yield point temperature or higher of thematerial of the second diffraction grating.

The second diffraction grating is cooled to the transformation pointtemperature or lower of the material of the second diffraction gratingwhile being contacted to the metal mold, and then, the metal mold isremoved.

The first diffraction grating is molded, using the molded seconddiffraction grating as a glass mold, at a molding temperature in a rangefrom the transformation point temperature or lower of the material ofthe second diffraction grating to the yield point temperature or higherof the material of the first diffraction grating.

The first and second diffraction gratings are cooled while beingcontacted to each other.

Referring now also to FIGS. 4A-F, an embodiment of a manufacturingmethod is described for manufacturing a defractive optical elementaccording to the first embodiment of present invention.

First, the diffraction grating is molded, which is one of the first andsecond diffraction gratings 2 and 3, and the material of which has ahigher yield point temperature At.

In this embodiment, since At2=392° C. and At3=566° C., the seconddiffraction grating 3 has a higher yield point temperature At.

A metal mold 50 having a reversal pattern of the second diffractiongrating 3, a material 3 a of the second diffraction grating 3, and ametal mold 51 having a predetermined surface are prepared (FIG. 4A). Amolding machine may be a typical glass molding machine for opticalelements.

Then, the metal mold 50, the material 3 a of the second diffractiongrating 3, and the metal mold 51 are heated to the yield pointtemperature At3 or higher of the material 3 a of the second diffractiongrating 3, and are pressed, so that the diffraction grating patternformed at the metal mold 50 is transferred to the material 3 a of thesecond diffraction grating 3 (FIG. 4B).

At this time, it is assumed that the inside of the molding machine is ina vacuum state, or is filled with inert gas. Next, the metal mold 50,the material 3 a of the second diffraction grating 3, and the metal mold51 are gradually cooled to the transformation point temperature Tg3 orlower of the material 3 a of the second diffraction grating 3, and then,the metal molds 50 and 51 are removed (FIG. 4C).

The second diffraction grating 3 is thus formed.

Then, the diffraction grating is molded, which is one of the first andsecond diffraction gratings 2 and 3, and the material of which has alower yield point temperature At. In this embodiment, the firstdiffraction grating 2 is molded.

The second diffraction grating 3, a material 2 a of the firstdiffraction grating 2, and a metal mold 52 having a predeterminedsurface are prepared (FIG. 4D).

The second diffraction grating 3, the material 2 a of the firstdiffraction grating 2, and the metal mold 52 are heated to a temperaturein a range from the yield point temperature At2 or higher of thematerial 2 a of the first diffraction grating 2 to the transformationpoint temperature Tg3 or lower of the material 3 a of the seconddiffraction grating 3, and then, are pressed, so that the diffractiongrating pattern formed at the second diffraction grating 3 istransferred to the material 2 a of the first diffraction grating 2 (FIG.4E).

The second diffraction grating 3, the material 2 a of the firstdiffraction grating 2, and the metal mold 52 are gradually cooled to thetransformation point temperature Tg2 or lower of the material 2 a of thefirst diffraction grating 2, and then, the metal mold 52 is removed(FIG. 4F). The first diffraction grating 2 is thus formed, and hence,the diffractive optical element 1 is completed.

One of the first and second diffraction gratings 2 and 3 may be moldedusing a metal mold, and the other may be molded using the moldeddiffraction grating as a glass mold.

In particular in this embodiment, the second diffraction grating 3 ismolded first, and the first diffraction grating 2 is molded using themolded second diffraction grating 3 as a mold (glass mold). Owing tothis, the materials 2 a and 3 a of the first and second diffractiongratings 2 and 3 are selected such that the difference between thetransformation point temperature Tg3 of the material 3 a and the yieldpoint temperature At2 of the material 2 a satisfies the condition:Tg3−At2>0.

If the above condition is not satisfied, the material of the seconddiffraction grating 3 serving as a mold when the first diffractiongrating 2 is molded may be melted. The pattern of the diffractiongrating may be changed, and the diffraction efficiency may decrease. Thechange in the grating pattern may be reduced if the difference betweenthe transformation point temperature Tg3 of the material 3 a of thesecond diffraction grating 3 and the yield point temperature At2 of thematerial 2 a of the first diffraction grating 2 is large. Thus, thedifference may be particularly Tg3−At2>50° C. More particularly, thedifference may be Tg3−At2>100° C.

In this embodiment, the material of the first diffraction grating 2employs the precision molding optical glass, named K-PG395 (Tg2=363° C.,At2=392° C.), manufactured by Sumita Optical Glass, Inc. The material ofthe second diffraction grating 3 employs the precision molding opticalglass, named K-VC80 (Tg3=530° C., At3=566° C.), manufactured by SumitaOptical Glass, Inc. At this time, Tg3−At2=138° C., and thus, thecondition (Tg3−At2>100° C.) is satisfied.

The molded second diffraction grating 3 is used as a mold when the firstdiffraction grating 2 is molded, so that the second diffraction grating3 and the material 2 a of the first diffraction grating 2 are heated,and cooled after the transferring, simultaneously. Since these parts aremade of glass members, the thermal expansion coefficients areapproximately the same, causing a fewer stress due to the differencebetween a linear expansion coefficient of the mold and that of themolding material. As a result, a deformation which may occur duringresin molding is minimized. A diffraction grating with an accuratepattern can be molded.

Since the second diffraction grating 3 is molded first, and then thefirst diffraction grating 2 is molded using the molded seconddiffracting grating 3 as a mold, if the strength of the seconddiffraction grating 3 is not sufficient, the second diffraction grating3 may be broken due to the pressure during the molding of the firstdiffraction grating 2. To avoid this, the thickness (distance betweenthe diffraction grating surface and a surface opposite thereto) of thesecond diffraction grating 3 may be 0.5 mm or greater, thereby providingthe sufficient strength.

Also, as shown in FIG. 5 (detailed illustration of FIG. 4E forabove-described the manufacturing method) showing the outer diameterrelationship of the diffractive optical element 1, the outer diameter ofthe previously molded second diffraction grating 3 may be smaller thanthe outer diameter of the first diffraction grating 2.

This provides a space to which the glass molding material protrudes whenthe first diffraction grating 2 is molded.

If the adhesion between interfaces of the materials 2 a and 3 a of thefirst and second diffraction gratings 2 and 3 is poor, an adhesion layermay be provided between the interfaces of the first and seconddiffraction gratings 2 and 3. Accordingly, the adhesion can be enhanced.

If the difference between refractive indices of the materials 2 a and 3a of the first and second diffraction gratings 2 and 3 is large, anantireflection layer may be provided between the interfaces of the firstand second diffraction gratings 2 and 3. Accordingly, the reflectivityat the interfaces can be reduced.

While the first and second diffraction gratings 2 and 3 have flatsurfaces (near the metal molds 51 and 52) located opposite to thediffraction grating surfaces, the surfaces do not have to be flat, andmay be spherical, aspherical, or curved.

In this case, one of the diffraction grating surfaces and the curvedsurface opposite thereto may be molded simultaneously. With thisconfiguration, the diffraction grating and the spherical, aspherical, orcurved surface can be molded simultaneously, facilitating themanufacturing.

Also, with this embodiment, the second diffraction grating 3 is heatedagain after it is molded. Accordingly, the refractive index can be morestable than through annealing. Thus, annealing does not have to beadditionally performed.

In each diffraction grating, if the distance (thickness) between thediffraction grating surface and the surface opposite thereto is large(0.5 mm or greater), the diffraction grating pattern can be moreaccurately transferred.

In this embodiment, one of the surfaces of the diffractive opticalelement may be provided with a refractive optical portion such as alens.

The design values in this embodiment are merely example values, and donot have to be the material and height of the diffraction gratings. Thesame can be said to the following embodiments.

Second Embodiment

A diffractive optical element according to a second embodiment of thepresent invention is described below.

In the first embodiment of the present invention, the diffractiveoptical element is contacting two-layer DOE. The second embodiment ofthe present invention provides a stacked diffractive optical element(DOE) 21 including three or more layers of diffraction gratings. FIG. 6is a cross section showing a primary portion of the second embodiment.

In this embodiment, a third diffraction grating is formed at least atone of surfaces opposite to the contacted grating surfaces of first andsecond diffraction gratings.

In particular, a third diffraction grating 24 is formed at a surfaceopposite to a grating surface of a first diffraction grating 22 having astructure the same as or alternatively similar to that of the firstembodiment. The third diffraction grating 24 has a grating pitchdistribution the same as or alternatively similar to those of the firstand second diffraction gratings 22 and 23. A period of correspondinggrating portions have a substantially uniform width. All layers of thediffraction gratings 22, 23 and 24 serve as one diffractive opticalelement.

Next, the diffraction efficiency of the diffractive optical element 21according to this embodiment is described.

For the stacked DOE in which two or more diffraction grating layers arestacked, the condition for the maximum diffraction efficiency ofcertain-order diffraction light is determined such that the differencebetween the optical-path length through the vertex of the gratingportion and the optical-path length through the root thereof (thedifference between the optical-path lengths passing through the vertexof the grating portion and that passing through the root thereof) isobtained for all diffraction gratings; the values are added; and thenthe sum is substantially an integral multiple of the wavelength.Accordingly, in the diffractive optical element 21 of this embodimentshown in FIG. 6, the conditional expression for the maximum diffractionefficiency of diffraction light with a design wavelength λ0 and adiffraction order m is as follows:±(n01−n02)d1±(1−n01)d2=mλ0  (5)where n01 is a refractive index with the wavelength λ0 of materials 22 aand 24 a of the first and third diffraction gratings 22 and 24, n02 is arefractive index with the wavelength λ0 of a material 23 a of the seconddiffraction grating 23, d1 is a grating height of the first and seconddiffraction gratings 22 and 23, d2 is a grating height of the thirddiffraction grating 24, and m is a diffraction order.

Herein, it is assumed that a light beam diffracted downward with respectto the zero-order diffraction light in FIG. 6 has a positive diffractionorder, whereas a light beam diffracted upward with respect to thezero-order diffraction light in FIG. 6 has a negative diffraction order.

Concerning the positive or negative sign of the grating heights in theexpression (5), it is assumed that the relationship between refractiveindices n01 and n02 of materials 22 a and 23 a of the diffractiongratings 22 and 23 is n01<n02. When the grating height of a gratingportions 22 b and 23 b increases from a lower portion to an upperportion in FIG. 6, the sign becomes negative.

In contrast, when n01>n02, and the grating height of the gratingportions 22 b and 23 b decreases from the lower portion to the upperportion in FIG. 6, the sign becomes positive.

With the structure in FIG. 6, when the relationship between therefractive indices n01 and n02 is n01>n02, the expression (5) can berewritten as follows:+(n01−n02)d1−(1−n01)d2=mλ0  (6)

With the structure in FIG. 6, a diffraction efficiency η(λ) with awavelength λ, which is not the design wavelength λ0, can be expressed asfollows:

$\begin{matrix}\begin{matrix}{{\eta(\lambda)} = {\sin\; c\;{2\left\lbrack {\pi\left\{ {M - {\left\{ {{\left( {{n\; 01(\lambda)} - {n\; 02(\lambda)}} \right)d\; 1} - {\left( {1 - {n\; 01(\lambda)}} \right)d\; 2}} \right\}/\lambda}} \right\}} \right\rbrack}}} \\{= {\sin\; c\;{2\left\lbrack {\pi\left\{ {M - {{\phi(\lambda)}\text{/}\lambda}} \right\}} \right\rbrack}}}\end{matrix} & (7)\end{matrix}$Herein, φ(λ) in the expression (7) can be expressed as follows:φ(λ)=−((n01(λ)−n02(λ))d1−(1−n01(λ))d2)d  (8)

In the above expressions, M is the order of diffraction light to beevaluated, n01(λ) is a refractive index with the wavelength λ of amaterial of the first diffraction grating 22, n02(λ) is a refractiveindex with the wavelength λ of a material of the second diffractiongrating 23, d1 is a grating height of the first and second diffractiongratings 22 and 23, and d2 is a grating height of a grating portion 24 bof the third diffraction grating 24.

Next, an implemental diffractive optical element as an example isdescribed below in more details to explain the feature of the secondembodiment. In this embodiment, materials 22 a, 23 a and 24 a of thefirst, second and third diffraction gratings 22, 23 and 24 employmolding glass.

The materials 22 a and 24 a of the first and third diffraction gratings22 and 24 employ glass molding material, named M-FDS910 (nd=1.821,νd=24.1, Tg (Tg22, Tg24)=455° C., At (At22, At24)=505° C.), manufacturedby Hoya Corporation. The material 23 a of the second diffraction grating23 employs glass molding low Tg optical glass S-LAL12 (nd=1.678,νd=54.9, Tg (Tg23)=562° C., At (At23)=600° C.), manufactured by OharaInc. It is assumed that the grating height d1 is 8.17 μm, and thegrating height d2 is 2.16 μm. Since the material of the diffractiongrating with a noticeably small Abbe number is used, the total gratingheight is smaller than that of the first embodiment.

FIG. 7 shows the characteristic of the diffraction efficiency of thediffractive optical element with the design order (positive firstorder), and the characteristics thereof with the zero order and thepositive second order, which are ±1 order with respect to the designorder.

The diffraction efficiency of the diffraction light with the designorder is 95.0% or higher for the entire visible wavelengths. Hence,flare, which is diffraction light with an unwanted order, can be 1.0% orlower for the entire visible wavelengths.

Next, a manufacturing method of the diffractive optical element 21 isdescribed below in details to explain the feature of the secondembodiment.

The manufacturing method of the diffractive optical element according tothe second embodiment is as follows.

First, the second diffraction grating is molded using a first metal moldat a molding temperature of the yield point temperature or higher of thematerial of the second diffraction grating.

The second diffraction grating is cooled to the transformation pointtemperature or lower of the material of the second diffraction gratingwhile being contacted to the first metal mold, and then, the first metalmold is removed.

The molded second diffraction grating is used as a glass mold, and asecond metal mold is used for the opposite surface of the seconddiffraction grating.

The first diffraction grating and the third diffraction grating oppositethereto are molded at a molding temperature in a range from thetransformation point temperature or lower of the material of the seconddiffraction grating to the yield point temperature or higher of thematerial of the first diffraction grating.

The first and second diffraction gratings and the second metal mold arecooled to the transformation point temperature or lower of the materialof the first diffraction grating while these are contacted, and thesecond metal mold is removed. The first and second diffraction gratingsare cooled while these are contacted to each other.

Referring now also to FIG. 8, an embodiment of a manufacturing method isdescribed for manufacturing the diffractive optical element according toa second embodiment of the present invention is described with referenceto FIG. 8.

First, the diffraction grating is molded, which is one of the first andsecond diffraction gratings 22 and 23, and the material of which has ahigher yield point temperature At when comparing the yield pointtemperature At22 with the yield point temperature At23.

In this embodiment, since At23>At24, the second diffraction grating 23is molded.

A metal mold (first metal mold) 53 having a reversal pattern of thesecond diffraction grating 23, a material 23 a of the second diffractiongrating 23, and a metal mold 54 having a predetermined surface areprepared (FIG. 8A).

Then, the metal mold 53, the material 23 a of the second diffractiongrating 23, and the metal mold 54 are heated to the yield pointtemperature At23 or higher of the material 23 a of the seconddiffraction grating 23, and are pressed, so that the diffraction gratingpattern formed at the metal mold 53 is transferred to the material 23 aof the second diffraction grating 23 (FIG. 8B).

The metal mold 53, the material 23 a of the second diffraction grating23, and the metal mold 54 are gradually cooled. These are cooled to thetransformation point temperature Tg23 or lower of the material 23 a ofthe second diffraction grating 23, and then, the metal molds 53 and 54are removed (FIG. 8C).

The second diffraction grating 23 is thus formed.

Then, the diffraction grating is molded, which is one of the first andsecond diffraction gratings 22 and 23, and the material of which has alower yield point temperature At.

In this embodiment, since At22=At24<At23, the first and thirddiffraction gratings 22 and 24 are molded.

The second diffraction grating 23, a material 22 a of the firstdiffraction grating 22, and a metal mold (second mold) 55 having areversal pattern of the third diffraction grating 24 are prepared (FIG.8D).

The second diffraction grating 23, the material 22 a of the firstdiffraction grating 22, and the metal mold 55 are heated to atemperature in a range from the yield point temperature or higher (At22or higher) of the material 22 a of the first diffraction grating 22 tothe transformation point temperature or lower (Tg23 or lower) of thematerial 23 a of the second diffraction grating 23, and are pressed, sothat the diffraction grating patterns formed at the second diffractiongrating 23 and at the metal mold 55 are simultaneously transferred tothe material 22 a of the first diffraction grating 22 (FIG. 8E).

The second diffraction grating 23, the first diffraction grating 22, andthe metal mold 55 are gradually cooled while being contacted to eachother, to the transformation point temperature Tg22 or lower of thematerial 22 a of the first diffraction grating 22, and then the metalmold 55 is removed (FIG. 8F).

Accordingly, the first and third diffraction gratings 22 and 24 aresimultaneously formed, and hence, the diffractive optical element 21 iscompleted.

The second diffraction grating 23 is molded first, and the first andthird diffraction gratings 22 and 24 are simultaneously molded using themolded second diffraction grating 23 as a mold (glass mold).

Assuming that At24 is a yield point temperature of the material 24 a ofthe third diffraction grating 24, and Tg3 a is a transformation pointtemperature of the material 23 a of the diffraction grating (seconddiffraction grating 23) which does not have a third diffraction grating24 at the surface opposite to the contacted surface, the followingcondition may be satisfied:At24<Tg3a

In particular, the material 22 a (24 a) of the first diffraction grating22 (third diffraction grating 24) and the material 23 a of the seconddiffraction grating 23 are selected such that the difference between thetransformation point temperature Tg23 of the material 23 a and the yieldpoint temperature At22 (At24) of the material 22 a (24 a) satisfies thecondition: At22(At24)<Tg23. That is, it is desirable thatTg23−At22(At24)>0.

If the condition is not satisfied, the material of the seconddiffraction grating 23 serving as a mold when the first diffractiongrating 22 is molded may be melted. The pattern of the diffractiongrating may be changed, and the diffraction efficiency may decrease.

The change in the grating pattern may be reduced if the differencebetween the transformation point temperature Tg23 of the material 23 aof the second diffraction grating 23 and the yield point temperatureAt22 of the material 22 a of the first diffraction grating 22 is large.Thus, the difference may be particularly Tg23−At22>50° C. Moreparticularly, the difference may be Tg23−At22>100° C.

In this embodiment, the material of the first diffraction grating 22employs the glass molding material, named M-FDS910 (Tg22=455° C.,At22=505° C.), manufactured by Hoya Corporation. The material of thesecond diffraction grating 23 employs the glass molding low Tg opticalglass S-LAL12 (Tg23=562° C., At23=600° C.), manufactured by Ohara Inc.At this time, Tg23−At22=57° C., and thus, the condition (Tg23−At22>50°C.) is satisfied.

Third Embodiment

A third embodiment of the present invention is described.

For the stacked DOE, the total grating thickness may be reduced bycombining materials with a large difference in their Abbe numbers. Thereare various kinds of materials for molding glass, and thus, adiffractive optical element having a small grating thickness may beformed.

FIG. 9 is a cross section showing a primary portion of a stackeddiffractive optical element (DOE) 31 having three-layer diffractiongratings.

The configuration of the diffractive optical element 31 according to thethird embodiment is described in details. In this embodiment, moldingglass is used for materials 32 a, 33 a and 34 a for forming first,second, and third diffraction gratings 32, 33, and 34.

The materials 32 a and 34 a of the first and third diffraction gratings32 and 34 employ precision molding optical glass, named K-PG325(nd=1.507, νd=70.5, Tg=288° C., At=317° C.), manufactured by SumitaOptical Glass, Inc.

The material 33 a of the second diffraction grating 33 employs precisionmolding optical glass, named K-PSFn2 (nd=2.002, νd=20.6, Tg=480° C.,At=514° C.), manufactured by Sumita Optical Glass, Inc. It is assumedthat the grating height d1 of grating portions 32 b and 33 b of thefirst and second diffraction gratings 32 and 33 is 3.37 μm, and thegrating height d2 of a grating portion 34 b of the third diffractiongrating 34 is 4.49 μm.

Since the material with a large Abbe number and the material with asmall Abbe number are used for the diffraction gratings, the totaldiffraction grating height may be further smaller than that of thesecond embodiment. Since the grating thickness is reduced, the lightincident on the grating wall face is reduced, and the flare componentcan be reduced.

In addition, the incidence angle characteristic of the diffractionefficiency can be improved, and hence, the configuration can be appliedto an optical system with a wide incidence angle.

Further, for manufacturing, a diffraction grating having a highlyaccurate pattern can be molded with glass.

FIG. 10 shows characteristic of the diffraction efficiency of thediffractive optical element 31 with the design order (positive firstorder), and the characteristics thereof with the zero order and thepositive second order, which are ±1 order with respect to the positivefirst order (design order).

The diffraction efficiency of the diffraction light with the designorder is 95.0% or higher for the entire visible wavelengths. Hence,flare, which is unwanted-order diffraction light, can be 1.0% or lowerfor the entire visible wavelengths.

The manufacturing method of the diffractive optical element 31 accordingto the third embodiment is similar to the second embodiment. The seconddiffraction grating 33 is molded first, and the first and thirddiffraction gratings 32 and 34 are simultaneously molded using themolded second diffraction grating 33 as a mold (glass mold).

Accordingly, the difference between the transformation point temperatureTg33 of the material 33 a of the second diffraction grating 33 and theyield point temperature At32 of the material 32 a of the firstdiffraction grating 32 may be Tg33−At32>0.

If the condition is not satisfied, the material 33 a of the seconddiffraction grating 33 serving as a mold when the first diffractiongrating 32 is molded may be melted. The pattern of the diffractiongrating may be changed, and the diffraction efficiency may decrease.

The change in the grating pattern may be reduced if the differencebetween the transformation point temperature Tg33 of the material 33 aof the second diffraction grating 33 and the yield point temperatureAt32 of the material 32 a of the first diffraction grating 32 is large.Thus, the difference may be particularly Tg33−At32>50° C. Moreparticularly, the difference may be Tg33−At32>100° C.

In this embodiment, the materials 32 a and 34 a of the first and thirddiffraction gratings 32 and 34 employ the precision molding opticalglass, named K-PG325 (Tg22−288° C., At22=317° C.), manufactured bySumita Optical Glass, Inc.

The material 33 a of the second diffraction grating 33 employs theprecision molding optical glass, named K-PSFn2 (Tg33=480° C., At33=514°C.), manufactured by Sumita Optical Glass, Inc. At this time,Tg33−At32=192° C., and thus, the condition (Tg33−At32>100° C.) issatisfied.

Fourth Embodiment

FIG. 11 is a schematic illustration showing a primary portion of anoptical system according to a fourth embodiment of the present inventionusing the diffractive optical element of the embodiment of the presentinvention. FIG. 11 is a cross section of an image-forming optical system(optical system) used in a digital camera, a video camera, or the like.

In FIG. 11, an imaging lens 101 has a refractive lens (refractiveoptical portion), an aperture stop 102, and the diffractive opticalelement 1 of the embodiment of the present invention. The aperture stop102 and the diffractive optical element 1 are provided in the refractivelens. An image plane 103 is a film or a charge coupled device (CCD). Thediffractive optical element 1 is an element having a lens function, andcorrects chromatic aberration of the imaging lens 101. Since thediffractive optical element 1 is made of molding glass as shown in anyof the first to third embodiments, the diffractive optical element 1 hasgood moldability and high environment resistance. In addition, theoptical system can be easily manufactured by manufacturing thediffraction gratings, and then bonding different refractive opticalelements at peripheral portions thereof. The optical system having goodproductivity can be provided.

While the diffractive optical element 1 has a flat glass plate as asubstrate and is provided near the aperture stop 102, it is not limitedthereto. The diffractive optical element 1 may have a lens as asubstrate, and may be provided on a concave surface, or a convex surfaceof the lens. In addition, a plurality of diffractive optical elementsmay be used in the imaging lens.

While this embodiment illustrates an imaging lens for a camera, it isnot limited thereto. The diffractive optical element of the embodimentof the present invention can be applied to an optical system for widewavelengths, such as an imaging lens of a video camera, an image scannerof a business machine, or a reader lens of a digital copier. This canprovide similar advantages.

Fifth Embodiment

FIG. 12 is a schematic illustration showing a primary portion of anoptical system according to a fifth embodiment of the present inventionusing the diffractive optical element of the embodiment of the presentinvention. FIG. 12 is a cross section of an observing optical systemsuch as a telescope or a binocular. In FIG. 12, an objective 104, animage inversion prism 105 for establishing an image, an eyepiece 106,and an evaluation plane (pupil plane) 107 are illustrated.

Reference numeral 1 denotes a diffractive optical element of theembodiment of the present invention. The diffractive optical element 1is provided for correcting chromatic aberration and the like generatedat the image plane 103 of the objective 104. If the diffractive opticalelement 1 is provided near an object with respect to the image plane103, the chromatic aberration only at the objective 104 can be reduced.Thus, the diffractive optical element 1 should be provided at least atthe objective 104 for an observing system that is observed with eyes.

Since the diffractive optical element 1 of the observing optical systemin FIG. 12 is made of molding glass as shown in any of the first tothird embodiments, the diffractive optical element 1 has goodmoldability and high environment resistance. Accordingly, high opticalperformance can be provided with reduced flare, and high resolution atlow frequencies, under various operating environments. In addition, theoptical system can be easily manufactured by manufacturing thediffraction gratings, and then bonding different refractive opticalelements at peripheral portions thereof. The observing optical systemhaving good productivity can be provided.

While the diffractive optical element 1 has a flat glass plate as asubstrate and is provided as shown in FIG. 12, it is not limitedthereto. The diffractive optical element 1 may have a lens as asubstrate, and may be provided on a concave surface, or a convex surfaceof the lens. Further, a plurality of diffractive optical elements of theembodiment of the present invention can be used.

In the observing optical system in FIG. 12, while the diffractiveoptical element 1 is provided at the objective 104, it is not limitedthereto. The diffractive optical element 1 may be provided at thesurface of the prism 105, or at a position inside the eyepiece 106. Thiscan provide similar advantages.

While this embodiment illustrates a binocular, it is not limitedthereto. The diffractive optical element may be applied to an observingoptical system, such as a terrestrial telescope, or a telescope forastronomical observation. This can provide similar advantages. Thediffractive optical element may be applied to an optical finder, such asa lens shutter camera, or a video camera. This can provide similaradvantages.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

This application claims the benefit of Japanese Application No.2007-084088 filed Mar. 28, 2007, which is hereby incorporated byreference herein in its entirety.

1. A diffractive optical element comprising: stacked first and seconddiffraction gratings made of different materials, wherein the materialsof the first and second diffraction gratings are glass, wherein thefirst and second diffraction gratings have grating surfaces contacted toeach other, wherein the materials satisfy the following conditions,Tg2≦600° C., andTg3≦600° C., wherein the materials satisfy one of the followingconditions,Tg3>At2, andTg2>At3, where Tg2 and At2 are a transformation point temperature and ayield point temperature of the material of the first diffraction gratingrespectively, and Tg3 and At3 are a transformation point temperature anda yield point temperature of the material of the second diffractiongrating respectively.
 2. The diffractive optical element according toclaim 1, wherein the first and second diffraction gratings are made ofthe materials satisfying one of the following conditions,Tg3−At2>50° C., andTg2−At3>50° C.
 3. The diffractive optical element according to claim 1,further comprising a third diffraction grating at least at one ofsurfaces of the first and second diffraction gratings, the surfacesbeing opposite to the grating surfaces contacted to each other.
 4. Thediffractive optical element according to claim 3, wherein the followingcondition is satisfied,At24<Tg3a where At24 is a yield point temperature of a material of thethird diffraction grating, and Tg3 a is a transformation pointtemperature of the material of the diffraction grating without the thirddiffraction grating at the surface opposite to the contacted gratingsurface.
 5. The diffractive optical element according to claim 1,wherein a total grating thickness of the first and second diffractiongratings is 20 μm or smaller.
 6. The diffractive optical elementaccording to claim 1, wherein an Abbe number νd of at least one of thematerials of the first and second diffraction gratings is 40 or smaller.7. The diffractive optical element according to claim 1, wherein an Abbenumber νd of at least one of the materials of the first and seconddiffraction gratings is 50 or greater.
 8. The diffractive opticalelement according to claim 1, wherein one of the first and seconddiffraction gratings is molded using a metal mold, and the otherdiffraction grating is molded using the molded one diffraction gratingas a glass mold.
 9. The diffractive optical element according to claim1, wherein a thickness of the second diffraction grating is 0.5 mm orgreater.
 10. The diffractive optical element according to claim 1,wherein an outer diameter of the second diffraction grating is smallerthan an outer diameter of the first diffraction grating.
 11. Thediffractive optical element according to claim 1, further comprising anadhesion layer between interfaces of the contacted grating surfaces. 12.The diffractive optical element according to claim 1, further comprisingan antireflection layer between interfaces of the contacted gratingsurfaces.
 13. The diffractive optical element according to claim 1,wherein at least one of the first and second diffraction gratings has acurved surface opposite to the contacted grating surface.
 14. Thediffractive optical element according to claim 13, wherein the onediffraction grating and the curved surface opposite to the gratingsurface are simultaneously molded.
 15. The diffractive optical elementaccording to claim 1, further comprising a refractive optical portion.16. An optical system comprising: the diffractive optical elementaccording to claim 1; and a refractive optical portion.
 17. Adiffractive optical element comprising: stacked first and seconddiffraction gratings made of different materials, wherein the materialsof the first and second diffraction gratings are glass, wherein thefirst and second diffraction gratings have grating surfaces contacted toeach other, wherein the materials satisfy the following conditions,Tg2≦600° C., andTg3≦600° C., wherein the materials satisfy one of the followingconditions,Tg2≠At3, andTg3≠At2, where Tg2 and At2 are a transformation point temperature and ayield point temperature of the material of the first diffraction gratingrespectively, and Tg3 and At3 are a transformation point temperature anda yield point temperature of the material of the second diffractiongrating respectively, and wherein an Abbe number νd of at least one ofthe materials of the first and second diffraction gratings is 50 orgreater.
 18. A diffractive optical element comprising: stacked first andsecond diffraction gratings made of different materials, wherein thematerials of the first and second diffraction gratings are glass,wherein the first and second diffraction gratings have grating surfacescontacted to each other, wherein the materials satisfy the followingconditions,Tg2≦600° C., andTg3≦600° C., wherein the materials satisfy one of the followingconditions,Tg2≠At3, andTg3≠At2, where Tg2 and At2 are a transformation point temperature and ayield point temperature of the material of the first diffraction gratingrespectively, and Tg3 and At3 are a transformation point temperature anda yield point temperature of the material of the second diffractiongrating respectively, and wherein one of the first and seconddiffraction gratings is molded using a metal mold, and the otherdiffraction grating is molded using the molded one diffraction gratingas a glass mold.